A bearing configured to support a main shaft of a turbo machine, such as a gas turbine or a turbocharger, is required to endure severe environments involving high temperature and high speed rotation. Focus has been given on a foil bearing as a bearing which is suitable for use under such severe conditions. The foil bearing has bearing surfaces formed of flexible thin films (foils) having low flexural rigidity. During rotation of the shaft, fluid films (for example, air films) are formed in bearing gaps formed between the shaft and the bearing surfaces of the foils, and the shaft is supported in a non-contact manner. With the foil bearing having the bearing surfaces formed of the foils, the bearing surfaces are allowed to be deflected, and the bearing surfaces are deformable along with, for example, displacement or thermal expansion of the shaft. Accordingly, there is an advantage in that the shaft can stably be supported even under severe conditions.
As an example of the foil bearing, a foil bearing described in JP 2015-113928 A (Patent Literature 1) is publicly known. In Patent Literature 1, there is disclosed the following thrust foil bearing. Specifically, two foil members each including a plurality of foils are overlapped with each other, and, while the foils of the foil member on an upper side and the foils of the foil member on a lower side are shifted by a half pitch, a portion of each foil on a downstream side in a rotation direction is arranged so as to be overlapped on a portion of an adjacent foil on an upstream side in the rotation direction. In the thrust foil bearing, the portion of each foil on the downstream side in the rotation direction functions as a top foil portion forming a bearing surface, and the portion of each foil on the side opposite to the rotation direction side functions as a back foil portion configured to support the top foil portion from behind.